二叉树的非递归遍历（非递归先序遍历，中序遍历，后序遍历）

主要利用栈的后进先出的特性

public class TraversalNonRecursive {

    /**
     * 先压头节点并弹出，然后先压右，后压左。这样弹出的顺序就是先左后右
     * @param head
     */
    public static void preOrder(TreeNode head){
        System.out.println("preOrder");
        if(head!=null){
            Stack<TreeNode> stack = new Stack<>();
            stack.push(head);
            while (!stack.isEmpty()){
                head=stack.pop();
                System.out.print(head.val+"  ");
                if(head.right!=null){
                    stack.push(head.right);
                }
                if(head.left!=null){
                    stack.push(head.left);
                }
            }
        }
    }

    /**
     * 将二叉树左边界依次入栈，直到为空，弹出，并使head等于右节点，此时如果head不为空，则压栈，为空则弹出。弹出的顺序刚好是中序遍历
     * @param head
     */
    public static void inOrder(TreeNode head){
        System.out.println("inOrder");
        if(head!=null){
            Stack<TreeNode> stack = new Stack<>();
            while (!stack.isEmpty()||head!=null){
                if(head!=null){
                    stack.push(head);
                    head=head.left;
                }else {
                    head=stack.pop();
                    System.out.print(head.val+"  ");
                    head=head.right;
                }
            }
        }
    }

    /**
     * 1.后序遍历顺序是左右中，而中右左的遍历很容易构造，跟先序遍历差不多，只是左右节点压栈顺序调换
     * 2.将中右左遍历压入一个辅助栈，再弹出，也就是后序的左右中遍历
     * @param head
     */
    public static void posOrder(TreeNode head){
        System.out.println("posOrder");
        if(head!=null){
            Stack<TreeNode> stack = new Stack<>();
            Stack<TreeNode> stackHelp = new Stack<>();

            stack.push(head);
            while (!stack.isEmpty()){
                head=stack.pop();
                stackHelp.push(head);
                if(head.left!=null){
                    stack.push(head.left);
                }
                if(head.right!=null){
                    stack.push(head.right);
                }
            }

            while (!stackHelp.isEmpty()){
                System.out.print(stackHelp.pop().val+"  ");
            }
        }
    }

    
    public static void main(String[] args) {
        TreeNode treeNode1=new TreeNode(1);
        TreeNode treeNode2=new TreeNode(2);
        TreeNode treeNode3=new TreeNode(3);
        TreeNode treeNode4=new TreeNode(4);
        TreeNode treeNode5=new TreeNode(5);
        TreeNode treeNode6=new TreeNode(6);
        TreeNode treeNode7=new TreeNode(7);

        treeNode1.left=treeNode2;
        treeNode1.right=treeNode3;
        treeNode1.left.left=treeNode4;
        treeNode1.left.right=treeNode5;
        treeNode1.right.left=treeNode6;
        treeNode1.right.right=treeNode7;

        preOrder(treeNode1);
        System.out.println("\n");
        inOrder(treeNode1);
        System.out.println("\n");
        posOrder(treeNode1);

    }
}

示例二叉树

打印结果